Everyone has put in their two cents and is valid, but I thought I'd put my opinion in as well. I work at an alignment shop and pack the bearings on these style hubs usually once a day and have never had any come back with a complaint. One thing not mentioned, but not pertinant to the howling, is you want to put some fresh grease on the spindle bearings ( where the axle shaft rides). I have never torqued the nuts, I just do it all by feel. As others have said tighten the inner spindle nut down while turning the hub, I just tighten it until it pretty tight with a ratchet to really seat the bearings, back it off and then retighten hand tight (basically with tighten ratchet with one finger and when it stops, thats it), Install the washer, if it doesnt index just as LONGHAIR said, flip the washer and check if not, barely loosen the nut and recheck, flip, recheck and so on till it goes on. Then once that is complete I tighten the outer nut pretty tight with the ratchet. Just check the hub when you are done, make sure that there is no play without it being too tight, a good indication that it is too tight is if it hard to rotate the hub with one hand. Good luck and hopefully the only howling you'll hear is the wind going past your window. Keep truckin.
Todd